1. Field of the Invention
The present invention relates to an image forming apparatus and image processing method, and more particularly, to an image forming apparatus and image processing method for optimizing an image formed on a prescribed recording medium by means of a digital halftoning method using a threshold value matrix.
2. Description of the Related Art
There is known an image forming apparatus which forms images on a recording medium, such as paper, by ejecting ink from nozzles toward the recording medium, while moving an inkjet head having an arrangement of a plurality of nozzles and the recording medium, relatively with respect to each other.
In an image forming apparatus of this kind, if there is a nozzle having a larger droplet ejection volume (or a smaller droplet ejection volume) than the other nozzles, then non-uniformity in density which is visible as banding occurs in the image formed on the recording medium.
In order to reduce density non-uniformity occurring due to droplet ejection errors in nozzles, in general, the image data is corrected on the basis of the droplet ejection characteristics of the nozzles. A specific example of a correction object is droplet ejection rate information in which the surface area of the density graduation information of the image data is modulated.
However, if the image data is corrected in order to reduce non-uniformities in density, then generally, the granularity of the image formed on the recording medium becomes worse.
Japanese Patent Application Publication No. 2000-307865 discloses that an upper limit and a lower limit are set for the correction of the image data, in such a manner that degradation of the granularity be restricted to a minimum. However, if the upper and lower limit values are provided for correction of the image data, then this places limitations on the correction of density non-uniformities, and therefore density non-uniformities may not be corrected sufficiently in cases where the droplet ejection error of a nozzle exceeds the limit of the correction.
Japanese Patent Application Publication No. 2004-168000 discloses a method in which, if there is a droplet ejection error, such as a droplet ejection volume larger than the ideal value in a nozzle, then dots of a normal density are switched to dots of lower density by ejecting droplets of a light-colored ink. However, if the normal density dots are switched to the low-density dots using droplet ejection of light-colored ink, then since this method is premised on the use of light-colored inks, it cannot be applied to image forming apparatuses which do not use light-colored inks. Moreover, in a method using light-colored inks of this kind, it is not possible to perform correction to increase the density of the dots.